As typical motion picture encoding system, there are those of two-dimensional discrete cosine transform (DCT) coding method and predictive coding method.
In the discrete cosine transform (DCT) coding method, signal power is concentrated in predetermined frequency components making use of the fact that picture signal has two-dimensional correlative a property. The resultant distribution of signal power frequency components is coded by expressing information to be transmitted with coefficients, and in this way frequency of information can be compressed.
For example, with a flat pattern portion, in which the motion picture signal is highly self-correlative, the discrete cosine transform coefficients (DCT coefficients) are distributed such that they are concentrated in low frequency components. Thus, in this case information to be transmitted can be expressed by merely coding the coefficients distributed such as to be concentrated in a low frequency region, thus permitting compression of information.
With the discrete cosine transform method, however, if discontinuous points of such signal as picture signal including contours (or edges) are to be transmitted by accurately expressing them with DCT coefficients, the generated DCT coefficients are distributed broadly from low to high frequency components. Therefore, an extremely large number of coefficients are required, thus-reducing the coding efficiency.
To solve this problem, a method of coarsening the coefficient quantizing characteristic or discarding high frequency component coefficients has been used for high compression coding motion pictures. These measures, however, are still insufficient in that the motion picture signal is deteriorated pronouncedly. For example, a distortion like a waver (which is called corona effect mosquito noise) is generated around the contour.
The predictive coding method, on the other hand, makes use, for coarsening the quantizing characteristic, of the eye's character that the brightness discrimination degree is low for contour portions of motion picture, and it has an advantage that it permits comparatively high compression coding. In this method, however, coarsening the quantizing characteristic for flat portions of motion picture, is liable to result in visually pronounced deterioration, such as generation of false contours or particle-like noises. Therefore, the predictive coding is not suited as means for high compression coding of flat portions.
Accordingly, it has been contemplated to let the discrete cosine transform (DCT) and predictive coding methods mutually make up for their drawbacks, that is, to switch the discrete cosine transform (DCT) and intra-block predictive or non-transform coding methods for each unit block according to the character of the pattern for high compression coding. More specifically, the discrete cosine transform (DCT) method may be used for a flat block of picture, while using the intra-block predictive coding (or NTC, the intra-block non-transform coding) for a contour portion of picture.
In the block-by-block intra-block predictive coding (NTC), there is a problem of block distortion which is generated when the quantizing is made coarsely. Specifically, this is a mosaic-like phenomenon produced for each block as a result of the coding.
In the intra-block predictive coding (NTC), quantizing errors generated as a result of coarse quantizing directly appear as brightness level changes (or deterioration). If great brightness level changes appear along the boundary between adjacent blocks, the shape of the blocks is pronounced visually like a mosaic.